Obtaining containers by blowing special preforms appropriately heated inside a mould of the desired shape is a method widely used in the packaging sector, in particular for making bottles for drinks.
Two basic methods exist which include on the one hand simple blowing and on the other, stretch-blowing, both of which provide for pneumatic blowing and mechanical stretching of the preforms in a mould. In both cases the preforms must reach the blowing or stretch-blowing machine in a thermal condition corresponding to the softening point of the material, so as to be plastically deformable inside the moulds.
Softening of preforms is performed in special ovens which include a series of heating modules positioned in series along a circuit of preforms.
To maximize both the heat efficiency of the oven and its dimensions, it is advisable for preforms to be inserted and slid inside the oven at as close a pitch as possible. (It is understood in this context that the term “pitch” refers to the spacing between one preform and an adjacent one.) This requirement is however contrary to the need for heated preforms to be sufficiently distanced for their insertion in the moulds of the blowing or stretch-blowing machine, the pitch of which, for obvious reasons of size, cannot be reduced beyond a certain limit.
In conventional machines, this distancing operation of the preforms to the pitch of the moulds is implemented by means of drive means, in particular distribution stars, which comprise a plurality of gripping means of preforms which are positioned at the same pitch as the moulds in the blowing machine or at an intermediate pitch between the latter and that of the preforms coming out of the oven. As a result, the distribution stars are placed in rotation at a higher speed than the supply speed of the heated preforms and lower or equal speed to the rotation speed of the blowing machine.
This change of speed imparts a considerable acceleration to the preforms. If one considers that preforms coming out of the oven are not rigid but softened, such acceleration may cause a deformation of the preforms, with a consequent formation of imperfections in the bottle obtained after blowing or stretch-blowing. In fact, deformed preform may contact the surface of the mould before the beginning of blowing, causing an abrupt cooling of the preform in the point of contact which is thus no longer able to undergo expansion.